Using spin-polarized density functional calculations, we have studied the interaction of carbon monoxide (CO) with bimetallic ConMn (n = 1-6) and ConMn6-n (n = 0-6) clusters. Various adsorption sites including atop, hollow, and bridge adsorption patterns and different possible spin states are considered. The CO molecule prefers to adsorb at the Co site rather than at the Mn site. Atop adsorption structure is energetically more favored over the hollow and bridge adsorption ones for the bimetallic clusters with an exception of Co5Mn. Large adsorption energy is found at Co3Mn, Co2Mn4, and Co3Mn3, associating with the relative stability of the bare Co Mn clusters and the electrostatic interactions as well as adsorption patterns. The activation of the C-O bond and the red shift of the C-O stretching frequency are sensitive to the adsorption sites and high chemical activity is identified for Co-6, Co5Mn, and Mn-6 clusters. More interestingly, the adsorption of CO has different influence on the magnetism of the clusters: the magnetic moment remains unchanged for CoMn and Co2Mn, while it is reduced by 2 mu(B) for ConMn (n = 3-6) and Co Mn6-n (n = 0-5) and is enhanced by 2 mu(B) for Mn-6 when a CO molecule is loaded to the cluster.